[DAIJOOBU]

It's one of the most used used phrase for conversation in Japan. It means 'All right, Fine, Okay'.

For example,

'DAIJOOBU DESUKA?' = Are you all right? / Are you Okay?

'DAIJOOBU DESU' = I'm fine / I'm all right

In Japanese, the subject is often omitted, so you don't need to say 'WATASHI(I)' and 'ANATA(you)'. If your friend is nervous or worried about something, you can tell them 'DAIJOOBU DAYO(casual form of DESU)', which means 'You'll be fine'.